HTML5 WebSockets revolutionize server communication by eliminating the need for AJAX requests. This tutorial guides you through creating a PHP WebSocket server and a client to exchange messages using the WebSocket protocol. A companion post covers using WebSockets with Node.js servers.

Understanding WebSockets

A WebSocket establishes a persistent, two-way communication channel between a client (like a browser) and a backend service. Unlike the request/response nature of HTTP, WebSockets support various protocols and enable server-to-client message delivery without constant polling.

What WebSockets Replace

WebSockets overcome the limitations of traditional HTTP communication. Prior to WebSockets, HTTP's statelessness made real-time communication challenging, as servers couldn't proactively push data to clients.

WebSockets also offer advantages over older techniques like AJAX long polling and Server-Sent Events (SSE). Long polling, while reducing latency by keeping connections open, still suffers from potential timeouts. This, along with the resource inefficiencies of many AJAX applications, highlights the need for a more efficient real-time solution – WebSockets. They enable server-side "push" technology, sending data to clients without needing a pre-established connection request.

Installing the Ratchet WebSockets Library

Ratchet is a PHP library for building real-time, bi-directional, event-driven applications using WebSockets. We'll use it to create our server.

Assuming Composer is installed, use this command:

composer require cboden/ratchet

This generates a composer.json file similar to this:

{
    "require": {
        "cboden/ratchet": "^0.4.4"
    }
}

Creating the WebSockets Server

Create server.php with the following code:

<?php
use Ratchet\MessageComponentInterface;
use Ratchet\ConnectionInterface;
use Ratchet\Server\IoServer;
use Ratchet\Http\HttpServer;
use Ratchet\WebSocket\WsServer;

require __DIR__ . '/vendor/autoload.php';

class WebSocketsServer implements MessageComponentInterface {
    protected $clients;

    public function __construct() {
        $this->clients = new \SplObjectStorage;
    }

    public function onOpen(ConnectionInterface $conn) {
        $this->clients->attach($conn);
        echo "New connection! ({$conn->resourceId})\n";
    }

    public function onMessage(ConnectionInterface $from, $msg) {
        foreach ($this->clients as $client) {
            if ($from !== $client) {
                $client->send($msg);
            }
        }
    }

    public function onClose(ConnectionInterface $conn) {
        $this->clients->detach($conn);
        echo "Connection {$conn->resourceId} has disconnected\n";
    }

    public function onError(ConnectionInterface $conn, \Exception $e) {
        echo "An error has occurred: {$e->getMessage()}\n";
        $conn->close();
    }
}

$server = IoServer::factory(
    new HttpServer(
        new WsServer(
            new WebSocketsServer()
        )
    ),
    8089
);

$server->run();

This code defines a WebSocketsServer class implementing MessageComponentInterface, handling connection (onOpen), message (onMessage), disconnection (onClose), and error (onError) events. The run() method starts the server on port 8089. (Note: The tutorial is missing the client-side code, which would be needed to complete the example.)

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